This invention is in the field of non-destructive quantitive material analysis using light. In particular, it is concerned with the reflectance of light from a material surface, and how the reflectance varies with wavelength. The constituents of a particular material may be known, but not their percentages in the material. A known technique of material analysis using reflected light is electroreflectance, in which a sample of the material is immersed in an electrolyte and subjected to a modulated electric field. At the same time, a light beam whose wavelength is varied in some predetermined manner is reflected from the sample onto a photodector. The curves thus obtained are indicative of the relative quantities of the constituents of the material in the sample. While this system gives extremely accurate results, it does have the disadvantages that it cannot be used for a sample in situ, nor can it be used to monitor a sample being processed. U.S. Pat. No. 4,142,802 shows such a known system.